Fix Python detection on Python 3.

Python 3 uses somewhat different naming of the library, making the custom
logic break. This just replaces all the custom Python-finding logic with
the built-in CMake Python-finding logic, which works and can be relied upon
to be updated as necessary.

cmake/Packages/Python.cmake

@@ -1,60 +1,9 @@
-FIND_PROGRAM(PYTHON_EXECUTABLE python
-      PATHS ENV PATH         # look in the PATH environment variable
-      NO_DEFAULT_PATH        # do not look anywhere else...
-      )
+find_package(PythonInterp)
+find_package(PythonLibs)
 
-IF(PYTHON_EXECUTABLE)
-  EXECUTE_PROCESS(COMMAND ${PYTHON_EXECUTABLE} -V
-    OUTPUT_VARIABLE STDOUT_VERSION
-    ERROR_VARIABLE PYTHON_VERSION
-    ERROR_STRIP_TRAILING_WHITESPACE)
-
-  IF(STDOUT_VERSION MATCHES "Python")
-    set(PYTHON_VERSION "${STDOUT_VERSION}")
-  ENDIF()
-
-  STRING(REGEX MATCH "([0-9]+)\\.([0-9]+)\\.?([0-9]*)"
-    PYTHON_STRIPPED_VERSION
-    ${PYTHON_VERSION})
-  STRING(REGEX MATCH "([0-9]+)\\.([0-9]+)"
-    PYTHON_STRIPPED_MAJOR_MINOR_VERSION
-    ${PYTHON_VERSION})
-
-  STRING(REPLACE "." "" PYTHON_VERSION_NO_DOTS ${PYTHON_STRIPPED_MAJOR_MINOR_VERSION})
-
-  EXECUTE_PROCESS(COMMAND ${PYTHON_EXECUTABLE} -c "import sys; sys.stdout.write(sys.prefix)"
-                  OUTPUT_VARIABLE PYTHON_ROOT)
-
-  FIND_LIBRARY(PYTHON_LIBRARY
-    NAMES python${PYTHON_VERSION_NO_DOTS} python${PYTHON_STRIPPED_MAJOR_MINOR_VERSION}
-    PATHS ${PYTHON_ROOT} ${PYTHON_ROOT}/lib
-    PATH_SUFFIXES lib
-    NO_SYSTEM_ENVIRONMENT_PATH
-    NO_DEFAULT_PATH
-  )
-  FIND_PATH(PYTHON_INCLUDE_DIR
-    NAMES Python.h
-    PATHS
-      ${PYTHON_ROOT}/include
-    PATH_SUFFIXES
-      python${PYTHON_STRIPPED_MAJOR_MINOR_VERSION}
-    NO_DEFAULT_PATH
-  )
-
-  IF(EXISTS "${PYTHON_INCLUDE_DIR}/Python.h" )
-    SET(PYTHON_FOUND TRUE CACHE BOOL "Python found successfully" FORCE)
-    MESSAGE(STATUS "Python found")
-    MESSAGE (STATUS "  * binary:   ${PYTHON_EXECUTABLE}")
-    MESSAGE (STATUS "  * version:  ${PYTHON_VERSION}")
-    MESSAGE (STATUS "  * includes: ${PYTHON_INCLUDE_DIR}")
-    MESSAGE (STATUS "  * libs:     ${PYTHON_LIBRARY}")
-  ELSE()
-    SET(PYTHON_FOUND FALSE CACHE BOOL "Python found successfully" FORCE)
-  ENDIF()
-
-  IF(PYTHON_FOUND)
+IF(PYTHON_FOUND)
     # Search for numpy
-    EXECUTE_PROCESS(COMMAND ${PYTHON_EXECUTABLE} -c "import numpy; print numpy.__version__"
+    EXECUTE_PROCESS(COMMAND ${PYTHON_EXECUTABLE} -c "import numpy; print(numpy.__version__)"
       RESULT_VARIABLE NUMPY_FOUND OUTPUT_VARIABLE NUMPY_VERSION OUTPUT_STRIP_TRAILING_WHITESPACE)
 #    IF(NUMPY_FOUND EQUAL 0 AND NOT NUMPY_VERSION VERSION_LESS 1.7)
 #      SET(NUMPY_FOUND TRUE)
@@ -70,13 +19,12 @@ IF(PYTHON_EXECUTABLE)
     IF(NUMPY_FOUND)
       SET(NUMPY_FOUND TRUE CACHE BOOL "Numpy found successfully" FORCE)
       EXECUTE_PROCESS(COMMAND ${PYTHON_EXECUTABLE} -c
-        "import numpy; print numpy.get_include()"
+        "import numpy; print(numpy.get_include())"
         OUTPUT_VARIABLE NUMPY_INCLUDE_DIR
         OUTPUT_STRIP_TRAILING_WHITESPACE)
       SET(NUMPY_INCLUDE_DIR ${NUMPY_INCLUDE_DIR} CACHE STRING "Numpy directory")
       MESSAGE (STATUS "  * numpy:    version ${NUMPY_VERSION}; headers ${NUMPY_INCLUDE_DIR}")
     ELSE()
       SET(NUMPY_FOUND FALSE CACHE BOOL "Numpy found successfully" FORCE)
     ENDIF()
-  ENDIF(PYTHON_FOUND)
-ENDIF(PYTHON_EXECUTABLE)
+ENDIF(PYTHON_FOUND)